This invention pertains to a multi leaf collimator for a beam of radiation the particles of which are selected from the group comprising high energy photons, electrons, protons and heavy ions to the extent not already covered by said U.S. Pat. No. 4,463,266.
Modern beam radiation technique makes use of high energy particles to irradiate deep seated tumours. High energy photons and electrons with energies in the order of 1 to 50 MeV are considered for radiation therapy.
In the neutron collimator described in said patent wedge-shaped slabs are arranged in opposed pairs such that the inner opposed edges of each pair of slabs are always directed towards the effective radiation source. This will prevent the formation of an undesired penumbra, that is a half shaded area extending in the longitudinal direction of motion behind each slab. Due to the wedge like shape of each slab also the plane main side surfaces of each slab will be directed towards the radiation source thereby also preventing the formation of an undesired penumbra in the direction perpendicular to the said longitudinal direction.
If high energy particles of the kind referred to above should be used the slabs of said neutron collimator are much to thick to absorb the particles and will generate undesirable scatter. Since the slabs are of solid tungsten and low carbon soft iron the known neutron collimator is heavy and requires a sturdy frame for supporting the slabs. This is a drawback from constructional point of view.
Moreover, if high energy particles of the kind referred to above in particular electrons are to be used in a collimator the air molecules within the collimator will interact with the particles of the radiation beam thereby giving rise to an effect appearing as if the effective radiation source is no longer, as desired, a point but rather an area of a certain extension. This is undesired since the particles emitted from the periphery of said area will cause an undesired scatter on the inner opposed edges and an increased penumbra resulting from the fact that neither the said inner edges nor the plane main side surfaces of the slab will point to the effective radiation source.
Should the known collimator be used for treatment with high energy photons said photons will collide with the atoms of the air giving rise to Compton interactions and secondary electrons, which will give rise to a substantial, non-desired increase of the dose to the skin of the patient.